The University of California, Berkeley, physicist once doubted the existence of climate change. Now he is convinced it's not only real but man-made, based on the latest results from his controversial review of temperature records.

"Call me a converted skeptic," Muller wrote in an op-ed published yesterday in The New York Times.

According to his Berkeley Earth Surface Temperature project, the average temperature on land has risen 1.5 degrees Celsius -- roughly 2.7 degrees Fahrenheit -- since 1753. That warming has tracked closely with the rise in greenhouse gas emissions from human activities, with little apparent contribution from natural factors like changes in solar activity, the group's latest paper concludes.

The researchers analyzed 14.4 million observations of average monthly land temperatures from 44,455 sites around the world, looking back about 100 years further and using five times more data than independent analyses by the National Oceanic and Atmospheric Administration, NASA and the U.K. Hadley Centre.

"Three years ago I identified problems in previous climate studies that, in my mind, threw doubt on the very existence of global warming," Muller wrote. "Last year, following an intensive research effort involving a dozen scientists, I concluded that global warming was real and that the prior estimates of the rate of warming were correct. I'm now going a step further: Humans are almost entirely the cause."

It is a marked about-face. Muller initially sought to investigate skeptics' claims that flawed data and methods had skewed prior examinations of global temperature trends. His startup kitty included $150,000 from a foundation started by oil billionaire Charles Koch that has supported efforts opposing mainstream climate change science.

'Stronger' results than IPCC
Even after the Berkeley project's initial findings last year agreed with prior analyses by NASA, NOAA and the Hadley Centre, Muller resisted pinning the blame for climate change on man-made greenhouse gas emissions.

"The amount that's due to humans is still open, and there are very big uncertainties in that," he said in November at a Washington, D.C., briefing organized by Democratic lawmakers (ClimateWire, Nov. 15, 2011).

Now Muller says Berkeley Earth's new results "are stronger than those of the Intergovernmental Panel on Climate Change," because they found solar activity had a "negligible" role in warming observed since the 1750s.

In its 2007 report, the IPCC concluded with 90 percent certainty that human-caused greenhouse gas emissions have been the primary factor in Earth's overall temperature rise since 1950. But the U.N. panel said natural causes, including changes in the sun's output, may have been a significant factor before then.

Still, the new findings are not likely to alter the polarized U.S. debate on climate change.

Derision from deniers
Muller's work has been rejected by climate skeptics, including some he once called collaborators. And mainstream climate researchers have greeted the Berkeley Earth research with exasperation, sometimes tinged with amusement (ClimateWire, Oct. 21, 2011).

Tonya Mullins, a spokeswoman for the Charles Koch Foundation, said it "has long supported and will continue to support sound, nonpartisan, scientific research."

"Our grants are designed to promote independent research," she said. "As such, recipients hold full control over their findings. In this support, we strive to benefit society by promoting discovery and informing public policy."

But several prominent skeptic blogs offered harsh criticism of Muller yesterday, even as they praised his former associate Anthony Watts.

Watts, who runs the skeptical blog "Watts Up With That?" has derided the Berkeley Earth results as "post-normal political theater" and "incomplete and rushed, non-quality controlled, error riddled" science. He has repeatedly criticized Muller for posting results on the Web before they have undergone formal peer review.

But yesterday, in a pointed move, Watts released his own non-peer-reviewed findings, which purport to show that poorly sited U.S. weather stations have exaggerated the magnitude of recent warming -- a claim Muller's Berkeley Earth group analyzed and rejected (as did a peer-reviewed, published analysis by scientists at the National Oceanic and Atmospheric Administration).

"Anthony Watts delivers devastating scientific blow to Muller's claims," screamed a banner headline on ClimateDepot, run by vocal skeptic Marc Morano. "Breaking: From BEST to worst in less than a day," began a post on the conservative Powerline blog.

Meanwhile, Judith Curry, a Georgia Institute of Technology climate scientist who co-authored the Berkeley Earth group's first four papers, said she declined to work on the latest analysis because of disagreements about Muller's methods.

"Their latest paper on the 250-year record concludes that the best explanation for the observed warming is greenhouse gas emissions," she told New York Time opinion blogger Andrew Revkin. "Their analysis is way oversimplistic and not at all convincing in my opinion."

Applause, and gentle mocking, from climate scientists
Michael Mann, the Pennsylvania State University climate scientist who created the "hockey stick" analysis of past temperatures, had a very different reaction.

"At this rate, Muller should be caught up to the current state of climate science within a matter of just a few years!" he tweeted yesterday.

Mann, a favorite target of climate skeptics, later published a fuller reaction on his Facebook page.

"There is a certain ironic satisfaction in seeing a study funded by the Koch Brothers -- the greatest funders of climate change denial and disinformation on the planet -- demonstrate what scientists have known with some degree of confidence for nearly two decades: that the globe is indeed warming, and that this warming can only be explained by human-caused increases in greenhouse gas concentrations," he wrote.

"I applaud Muller and his colleagues for acting as any good scientists would, following where their analyses led them, without regard for the possible political repercussions (they are of course almost certain to be attacked by climate change deniers for their findings)."

In the meantime, the Berkeley Earth group is working to publish its findings, said Elizabeth Muller, Richard Muller's daughter and the project's executive director.

The group has submitted the new paper, and its four previous studies, to the Journal of Geophysical Research-Atmospheres, she said.

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